1. Field of the Invention
The present invention relates to a solid-state image sensing device suitable for a distance-measuring sensor.
2. Description of the Prior Art
FIG. 7 is a view for describing the principle of distance measurement with the use of a distance measuring sensor. The distance measuring sensor comprises a lighting unit 2, a solid-state image sensing device 3 and a processing circuit 4. An object (subject) 1 is irradiated with an irradiating infrared pulse 10 emitted from the lighting unit 2, and a light 12 which is the irradiated infrared pulse 10 reflected by the object 1 is incident on the solid-state image sensing device 3. Then, the solid-state image sensing device 3 stores an information charge in response to the reflected light 12 and sends the information to a processing circuit 4. The processing circuit 4 calculates, on the basis of the information sent from the solid-state image sensing device 3, the phase difference between the irradiating infrared pulse 10 and the reflected light 12, and calculates therefrom the distance between the sensor and the object 1. A specific method for measuring the distance with the distance-measuring sensor will be now described below.
A CCD (a charge coupled device) solid-state image sensing device is a charge transfer element which can transfer information charge as one signal packet at a rate synchronizing with an external clock pulse in one direction in orderly sequence. FIG. 8 is a plan view showing an outline configuration of a frame transfer type of solid-state image sensing device 3. The frame transfer type of solid-state image sensing device 3 has an image sensing portion 3i, a storing portion 3s, a horizontal transfer portion 3h and an output portion 3d. The image sensing portion 3i includes vertical shift registers comprising a plurality of shift registers extended in a vertical direction (a longitudinal direction in FIG. 8) in parallel with each other, and each bit of each shift register is arranged as a matrix of two dimensions. The storing portion 3s also comprises the vertical shift registers comprising the several shift registers extended in the vertical direction (the longitudinal direction in FIG. 8) in parallel with each other. The vertical shift register included in the storing portion 3s is shaded, and each bit of each shift register functions as a storing picture element for storing information charge. A horizontal transfer portion 3h comprises horizontal shift registers arranged so as to extend to a horizontal direction (a transverse direction in FIG. 8). The output of each shift register of the storing portion 3s is connected to each bit of the horizontal shift register. The output portion 3d comprises a capacitor for temporarily storing the information charge transferred from the horizontal shift register in the horizontal transfer portion 3h, and a reset transistor for emitting the information charge stored in the capacitor.
FIG. 9A is a schematic plan view showing one part of a conventional image sensing portion 3i, and FIG. 9B is a side sectional view along the A-A line thereof. In FIG. 9B, a P well (PW 21) is formed in an N-type semiconductor substrate (an N-sub) 20, and an N well (NW) 22 is formed thereon. Specifically, on the N-type semiconductor substrate 20, a P well (PW 21) doped with a P-type of impurity (dopant) is formed. In the surface region of the P well 21, the N well 22 doped with a high concentration of an N-type dopant is formed.
In addition, in order to separate the channel regions of vertical shift registers, a separation region 25 is installed. By ion-implanting a P-type of dopant in an N well 22 in parallel to each other at predetermined intervals, the separation region 25 comprising a p-type impurity region is formed. The N well 22 is electrically partitioned by the adjacent separation regions 25, and the region sandwiched by the separation regions 25 becomes a channel region which is a transfer path of information charge. The separation regions 25 form potential walls between the adjacent channel regions, and electrically separate each channel region.
On the surface of a semiconductor substrate 20, an insulation film 23 is formed. So as to be perpendicular to the extending direction of a channel region, a plurality of transfer electrodes 24 made of a polysilicon film are arranged in parallel to each other, through the insulation film 23. Furthermore, in order to reduce the resistant component of the transfer electrodes 24, backing wires 26 consisting of a tungsten silicide film are connected through an opening to the predetermined number of the transfer electrodes 24, and are arranged in parallel to the extending direction of the channel regions. A group of five adjacent transfer electrodes 24-1, 24-2, 24-3, 24-4 and 24-5 corresponds to one picture element.
FIG. 10 shows the relationship, in the distance-measuring sensor shown in FIG. 7, during an image sensing period Ts when an irradiating infrared pulse 10 has irradiated an object 1 from a lighting unit 2, among an irradiating infrared pulse 10, the intensity of a reflected light 12, the transfer clock signal of a solid-state image sensing device 3 and the stored charge of the solid-state image sensing device 3. In FIG. 10, a horizontal axis shows the time.
At first, in advance of an image sensing period Ts, the information charge stored in each picture element of a solid-state image sensing device is removed by a method such as an electronic shutter.
Subsequently, as shown in FIG. 10(a), an irradiating infrared pulse 10 having the brightness modulated into a predetermined frequency consisting of an irradiation period Tp and a non-irradiation period Tp, is emitted from a lighting unit 2 of a distance-measuring sensor toward an object 1 only during an image sensing period Ts. At this time, as shown in FIG. 10(b), the object 1 reflects a light having a phase difference (a phase delay) in proportion to the distance between the distance-measuring sensor and the object 1, and the reflected light 12 is incident on a solid-state image sensing device 3. When representing the distance between the distance-measuring sensor and the object 1 as r, and the light velocity as c, an optical path length is 2r and one cycle is 2 Tp, so that the phase difference between them is expressed by 2πr/c/Tp [rad]. Here, as shown in FIG. 10(c), by using a transfer clock signal, the solid-state image sensing device 3 synchronizes a charge-storing period (an ON period) and a charge non-storing period (an OFF period) to the irradiating infrared pulse 10. When the device is sensing an image, a potential profile in an N well 22 along the channel region (a side cross section along a B-B Line in FIG. 9A) of an image sensing portion 3i is shown in FIGS. 11A and B. In a charge-storing period, the device turns three transfer electrodes 24-2 to 24-4, among a set of transfer electrodes 24 as shown in FIG. 11A, to an ON state to form a potential well 30 in the channel regions under the transfer electrodes 24-2 to 24-4, and turns the remaining transfer electrodes 24-1 and 24-5 to an OFF state. As a result, the potential well 30 under the transfer electrodes 24-2 to 24-4 in the ON state stores an information charge corresponding to the intensity of a reflected light 12. At this time, as shown in FIG. 10(d), in the period when the charge-storing period overlaps with the high intensity period of the reflected light, an ON-time charge is stored. In the charge non-storing period, as shown in FIG. 11B, the image sensing device turns one transfer electrode 24-3 among a set of the transfer electrodes 24 to the ON state, to form a potential well 31 in the channel region under the transfer electrode 24-3, and turns the remaining transfer electrodes 24-1, 24-2, 24-4 and 24-5 to the OFF state. As a result, the potential well 31 under the transfer electrode 24-3 in the ON state retains the information charge by that time. However, during the charge non-storing period as well, the OFF-time charge is stored in the period when the charge non-storing period overlaps with the high intensity period of the reflected light, as shown in FIG. 10(d). This is because, as is clear in FIG. 11B, in the charge non-storing period as well, the reflected light 12 is incident on the transfer electrode 24-3 and generates a new information charge. Because the ON-time charge and the OFF-time charge, generated in one cycle, are extremely low, the charge-storing period and the non-charge-storing period are repeated only during the imaging period Ts, and then each picture element cumulatively stores the ON-time charge and the OFF-time charge therein in every one cycle as the information charge.
After the lapse of an image sensing period Ts, an image sensing device transfers information charge stored in each picture element by that time, to a storing portion 3s from an image sensing portion 3i. When the information charge is transferred, as shown in a transferring period Tt in FIG. 12, five-phases of transfer clock signals φ1 to φ5 are applied to every combined unit consisting of five adjacent transfer electrodes 24-1, 24-2, 24-3, 24-4 and 24-5, the potentials of channel regions under the transfer electrodes 24-1, 24-2, 24-3, 24-4 and 24-5 are controlled, and the information charges are transferred to one direction.
As described above, an information charge stored in the picture element of a solid-state image sensing device 3 when the solid-state image sensing device 3 makes an irradiating infrared pulse 10 irradiate an object 1 only during an image sensing period Ts, and sets a charge-storing period so as to synchronize with the irradiating infrared pulse 10, is defined as Qp. On the other hand, the information charge stored in the picture element of the solid-state image sensing device 3 when the solid-state image sensing device 3 makes the irradiating infrared pulse 10 continuously irradiate the object 1 only during the image sensing period Ts, and sets the whole image sensing period Ts as a charge-storing period of each picture element, is defined as Qs. Then, because the quantity of the generated information charge per unit time is proportional to a photoreceptive area, an OFF-time charge becomes one-third of an ON-time charge. Consequently, the ratio of the information charge Qp to the information charge Qs is described below according to FIG. 10(d).Qp/Qs={(Tp−2r/c)+(2r/c)·(1/3)}/2Tp  (1)If the expression (1) is rewritten, the distance r between a distance-measuring sensor and an object 1 is expressed by the following expression.r=3Tp·c(1−2Qp/Qs)/4  (2)
As described above, the distance r between a distance-measuring sensor and an object 1 is calculated from the expression (2). Practically, information charges Qp and Qs cannot be directly measured, so that the distance r is calculated from the ratio of a picture signal output from an output portion in correspondence with the information charge Qp obtained when the irradiating infrared ray is applied in a pulse form, and the information charge Qs obtained when continuously applied.
As described above, the distance r between a distance-measuring sensor and an object 1 is calculated from the expression (2). However, besides the fact that an ON-time charge per unit cycle is extremely small, this method generates a not small OFF-time charge in a charge non-storing period as well, consequently makes the difference of sensitivity between a charge storing period and the charge non-storing period small in a solid-state image sensing device 3, and causes a problem that an error in calculating the distance r increases. Here, though the distance r is calculated from the ratio of information charge Qp during pulse irradiation to information charge Qs during continuous irradiation, the rate of change (a differential coefficient) of the ratio by the distance r is −2/(3Tp·c).